1. Field of the Invention
The present invention relates to a technique for controlling the strip thickness in a rolling mill, and more particularly to a method and a system for controlling the strip thickness, which aims at reducing to an appreciable extent the gauge deviation of the strip at the head and tail ends of the strip inherent to the prior art method and system.
2. Description of the Prior Art
In conventional methods for controlling the strip thickness in a rolling mill, the thickness deviation of the delivered or outgoing strip is detected by the gauge meter method, the mass flow method, or the thickness meter method and the screwdown or press-down position of the work roll and/or the speed of the work rolls are so controlled that the thickness deviation may be reduced to zero.
These methods, using feedback control, have a drawback that the accurate control of strip thickness is difficult due to the delays in the detection of the strip thickness and in the control operation of the control system. As one of the conventional methods for eliminating such a drawback, there has been proposed a strip feed-forward control in which the strip thickness deviation is detected at the entrance side of the rolling mill, the thickness of the delivered strip is estimated on the basis of the strip thickness deviation at the entrance side, and the press-down position of the work rolls and/or the speed of the work rolls are so controlled as to reduce the estimated delivered strip thickness deviation to zero.
A typical example of the feed-forward control will be described. The output signal of the thickness meter provided at the entrance side of a stand of a rolling mill is delayed by a time T.sub.L, and press-down position of the work roll is corrected. Here, the time T.sub.L is given by the following expression EQU T.sub.L =L/v (1),
where L is the distance from the position of the thickness meter to the work roll of the mill stand, and v is the velocity of the strip. It is considered to make a point of detection coincide with the point of control by delaying the moment of correction in the press-down of the work roll by the feed time required for the point of detection to shift from the position of the thickness meter to the work roll. Such a technique is disclosed, for example, in Japanese Patent Publication No. 25509/63. By further developing this technique, it is known to control the interstand tension when the point of detection reaches a predetermined mill stand. The method for controlling the interstand tension is disclosed, for example, in Japanese Patent Publication No. 7140/76 (based on U.S. patent application Ser. No. 92,349). Since the delay in the operation of the press-down apparatus is greater than the delay in the rolling speed, the above method employs a means for applying to the press-down apparatus a press-down position correcting signal based on the detected strip thickness by shortening the delay time T.sub.L by the delay T.sub.S in response. Moreover, since the press-down apparatus has a delay in response, the strip thickness control using tension (velocity) control which has a more rapid response, and like control methods have been proposed and put into practice. It is therefore clear that if these methods are combined with the feedback method, the accuracy in strip thickness can be much improved.
However, concerning the off-gauge portions of the strip at the head and tail ends thereof which causes a problem in the control of strip thickness, the lengths of the off-gauge portions are considerably reduced according to the control methods as described above, but at present the methods can not be said to be satisfactory. A certain steel producing factory reported that steel strips which were off-gauge portions and which could not be used as commercial product, amount to 450 tons per month and that the amount of the off-gauge portions was about 0.6% of the total amount of production by the factory. It is therefore easily understood that the reduction of the useless amount results in various improvements such as the reductions of production cost, energy consumption etc. The inventors, therefore, determined why the off-gauge portions were generated and have found a problem inherent to the prior art feed-forward control. The problem will be described below, with the above-described feed-forward control for correcting the press-down position taken as an example. Recently, the strip thickness control usually uses a computer. The thicknesses (strip thickness) or thickness deviations are taken into the computer at a predetermined sampling rate so that the degree of press-down according to the thickness or thickness deviation is determined to control the press-down position. The delay in the response of the press-down apparatus can be compensated to a certain extent, as described above, by advancing the timing of press-down control by the time of delay in the operation of the press-down apparatus and therefore it is assumed for simplicity that the press-down apparatus has no delay in response. Now, the strip thickness is detected at a time, the correction amount for the press-down position according to the detected thickness, and the press-down apparatus is manipulated in accordance with the correction amount. If the operation of the press-down apparatus takes place when the detected portion reaches the mill stand, the detected portion can be controlled very accurately. However, the points to be subjected to detection lie at discrete positions along the length of the strip and the control system is constructed on the assumption that the strip thickness is constant during sampling periods. Namely, since the strip thickness to be detected is constant during the sampling period, control is made under the condition that there is a stepwise variation in strip thickness. However, the actual variation in the strip thickness is not stepwise but continuous. The sampling value usually varies from one sampling epoch to another so that the press-down operation for one sampling value is often opposite in direction to that for another. In this type of control, the fluctuation of the control amount due to the delay in response or the response characteristic will be considerable in a practical rolling operation. It is clear that if control corresponding to the case where there is no fluctuation of strip thickness over a certain interval (or period), is made for the case where the strip thickness varies continuously, then such a control will be insufficient. With such a conventional strip thickness control, it is difficult to obtain the on-gauge thickness in a series of rolling processes: threading-acceleration-normal rolling-deceleration-drawing, except in the portion under the normal rolling where tension, rolling speed and the thickness of oil film between roll and strip are constant. Accordingly, it is easily understood that off-gauge portions are generated at the head end of the strip in "threading and acceleration" steps and at the tail end of the strip in "deceleration and drawing" steps.
The present invention, made to correct the deficiencies described above, aims at providing a novel method and a system for controlling the strip thickness in a rolling mill, according to which the length or the rate of off-gauge portions at the ends of strip can be minimized.
According to one feature of the present invention, the thickness deviation of delivered strip can be minimized by the control taking in consideration the change with time in strip thickness deviation or dynamic characteristic.
Other features and objects of the present invention will become obvious from the following description.